In a recent study published in the International Journal of Obesity, researchers in China conducted a systematic review and meta-analysis to investigate the association between body mass index (BMI) and semen quality. They found that obesity and overweight are significantly associated with reductions in semen quality, with more pronounced effects in men with higher levels of obesity.
Study: Association between body mass index and semen quality: a systematic review and meta-analysis. Image Credit: Ljupco Smokovski / Shutterstock
Background
Infertility affects about 15% of reproductive-age couples globally, with 20–70% of cases attributed to male factors, primarily poor semen quality. Research shows a global decline in semen quality, with significant decreases in total sperm number and sperm concentration observed from 1973 to 2011 in Western countries and similarly in China from 1981 to 2019. Factors such as age, lifestyle, and environmental influences contribute to this decline. Abnormal BMI is a potential risk factor, given its global increase. Although several meta-analyses link obesity to declines in semen parameters, results are inconsistent due to methodological limitations, including varying BMI classifications and potential biases. The association between BMI and semen quality across different fertility statuses requires further investigation. Therefore, in the present study, researchers performed a systematic review and meta-analysis to examine the link between BMI and semen quality while quantifying the impact of obesity and overweight across individuals with different fertility statuses and locations.
About the study
A total of 50 human studies were identified from databases including Embase, PubMed, and Web of Science. They constituted data on 71,337 men in the age group 26–44 years. Studies were included if they categorized BMI into the following groups: reference group (<25.0 kg/m²), overweight (25.0–29.9 kg/m²), and obese (≥30.0 kg/m²) or could be transformed into these categories. Additionally, the studies reported semen parameters across BMI categories, such as semen volume, sperm concentration, total motility, total sperm number, progressive motility, and normal forms. Excluded studies were reviews, conference abstracts, letters, experimental studies, and studies without quantitative data on semen parameters and BMI. Data were extracted, including study details, population characteristics, BMI ascertainment, semen quality evaluation guidelines, semen collection, and semen parameters by BMI category. Based on men's fertility status, populations were classified as general population, infertile or sub-fertile men, and suspiciously sub-fertile men.
A modified Newcastle-Ottawa Scale (NOS) was used to assess the risk of bias in the studies. Further, various statistical methods were used, including the random effects model, Cochran's Q tests, I² statistics, Egger's test, subgroup analyses, and meta-regression.
Results and discussion
A majority of the studies were found to focus on suspiciously sub-fertile men (n = 25), while others addressed the general population (n = 11) and infertile or sub-fertile men (n = 14). BMI was assessed on-site in 25 studies and self-reported in five of them. Semen quality was primarily evaluated using guidelines from the World Health Organization (1999 and 2000). Further, most studies were found to collect semen once (n = 41). A total of 30 studies were found to be at a low risk of bias, and 20 were found to be at a high risk.
Men with obesity showed significant reductions in semen parameters compared to the reference group, including decreases of 0.24 ml in semen volume, 19.56 × 10⁶ in total sperm number, 2.21% in total motility, 5.95% in progressive motility, and 1.08% in normal forms (all p < 0.05). In contrast, men who were overweight showed smaller reductions in semen volume (0.08 ml) and progressive motility (2.91%), with no significant changes in other parameters. As per the study, class II/III obesity led to more significant declines in semen quality than class I obesity, with significant heterogeneity across studies for most parameters analyzed.
Men with obesity showed greater declines in semen quality compared to those with overweight, with significant reductions observed across different fertility statuses and assessment methods. The impact varied according to the guidelines used for semen evaluation and study location. Significant reductions in semen quality were noted across various regions (America, Asia, Europe) for men with obesity. No publication bias was found in various semen parameters for men with obesity or overweight, except for total sperm number in overweight men.
The study is strengthened by its large sample size, comprehensive data synthesis through unified BMI classifications, and analysis across different fertility statuses. However, the study is limited by potential inaccuracies in aggregated data, bias from using underweight and normal weight as references, unaccounted confounding factors, BMI's inability to distinguish muscle from fat, and reliance on conventional semen parameters that may not fully reflect sperm function.
Conclusion
In conclusion, the study found a significant association between obesity, being overweight, and declines in semen quality. The link was particularly more pronounced in men with class II/III obesity compared to class I. The findings emphasize the importance of maintaining a normal weight to improve semen quality and male reproductive health. In the future, studies could further explore the effects of obesity on male reproduction using comprehensive indicators.
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